Carlos Vázquez-Vázquez

GIANT MAGNETORESISTANCE IN FINE PARTICLE OF LA0.67CA0.33MNO3 SYNTHESIZED AT LOW TEMPERATURES

Using a sol‐gel method we have prepared monodispersive particles of La0.67Ca0.33MnO3 in the range from 20 to 110 nm at temperatures from 540 to 1000 °C. A magnetoresistance above 10% was obtained in a field of 1 kOe for all the particles sizes. These results may be relevant for future applications.

Characterization of La0.67Ca0.33MnO3±δ particles prepared by the sol–gel route

We report in this work the characterization of La0.67Ca0.33MnO3±δparticles synthesizedvia sol–gel technology starting from an aqueous solution of the metallic nitrates and using urea as gelificant agent. The gelification is assumed to happen through the formation of polynuclear species by condensation reactions between hydroxo complexes. Gels were decomposed at 250 °C and calcined for 3 h at temperatures ranging from 300 to 1000 °C. Complete crystallization takes place at ca. 600 °C. The powders were structurally characterized by X-ray diffraction and their structural parameters were calculated using the Rietveld method. The MnIV content of the several samples was determined to be higher than the stoichiometric 33%. TEM micrographs show elongated particles of which the polar (long) axis size increases from 40 to 300 nm as the calcination temperature increases. Magnetization and magnetoresistance studies are reported showing that the particles smaller than 80 nm behave as single magnetic domains while the large ones behave as multidomains. A magnetoresistance of 12% at 1 kOe was observed for all the particles synthesized by this sol–gel method.

Synthesis of small atomic copper clusters in microemulsions.

We report evidence of the formation of small atomic copper clusters, Cu(n), by the microemulsion technique, and how their size can be controlled by adjusting the percentage of the reducing agent used. Copper clusters were characterized by UV-visible spectrophotometry and atomic force microscopy. Photoluminescent copper clusters, Cu(n), with n less, similar 13, can be obtained using very low percentages of the reducing agent (<10% of the stochiometric amount). Photoluminescent clusters disappear for larger percentages of reducing agent, giving rise to larger copper clusters (0.8-2.0 nm), showing a red-shift of their UV-visible absorption bands as they grow in size. Finally, by using near stoichiometric amounts, nanoparticles of 2.9 +/- 1.1 nm in size, displaying the characteristic plasmon band, can be obtained.

Magnetic properties of chromium (III) oxide nanoparticles

Cr2O3 nanoparticles of controlled particle size were prepared by calcination of a precursor, Cr(OH)3, obtained by precipitation with sodium hydroxide. Samples were characterized by transmission electron microscopy and x-ray diffraction. Average particle sizes ranged from 20 to 200 nm. The magnetic properties of Cr2O3 nanoparticles show the presence of a net magnetic moment at the surface due to the large surface/volume ratio. This fact modifies the classical behaviour expected for bulk antiferromagnetic particles. Below the Neel temperature, magnetization curves as a function of the applied magnetic field show the presence of coercive forces in the low-field range.

Synthesis of atomic gold clusters with strong electrocatalytic activities.

Small atomic gold clusters in solution, Au n , stabilized by tetrabutyl ammonium bromide (TBABr), have been synthesized by a simple electrochemical technique, based on the anodic dissolution of a gold electrode in the presence of TBABr salt, and using acetronitrile as solvent. The presence of clusters in the range Au3-Au11 were detected by MALDI-TOF spectroscopy, and further characterized by UV-vis absorption spectroscopy, TEM, AFM, X-ray diffraction, and cyclic voltammetry. Clusters display a semiconductor behavior with a band edge of approximately 2.5 eV. We report here their extraordinarily high electrocatalytic activity toward the O2 reduction reaction in acid solutions, which can explain Zhangs results, showing that a four-electron mechanism seems to occur because of the facile reduction of H2O2 on gold clusters compared to bulk gold or larger gold nanoparticles.

Dependence of the magnetic properties Gd2−xCexCuO4, 0≤x≤0.15, on their particle size

Gd2−xCexCuO4, 0≤x≤0.15, have been synthesized by the solid state reaction, the sol‐gel technique and the water‐in‐oil microemulsion, in order to vary the particle size. The samples have been characterized by x‐ray diffraction, photon correlation spectroscopy and transmission electron microscopy. Magnetization and ac susceptibility measurements were performed between 4 and 300 K. The extrapolated linear part of the inverse magnetization shifts to lower temperatures as the particle size decreases. The internal field associated to the WF of the Cu moments diminishes with the particle diameter. Differences between the zero‐field‐cooling and the field‐cooling magnetization curves were observed for T

Self-Assembly of Silver Metal Clusters of Small Atomicity on Cyclic Peptide Nanotubes

Subnanometric noble metal clusters, composed by only a few atoms, behave like molecular entities and display magnetic, luminescent and catalytic activities. However, noncovalent interactions of molecular metal clusters, lacking of any ligand or surfactant, have not been seen at work. Theoretically attractive and experimentally discernible, van der Waals forces and noncovalent interactions at the metal/organic interfaces will be crucial to understand and develop the next generation of hybrid nanomaterials. Here, we present experimental and theoretical evidence of noncovalent interactions between subnanometric metal (0) silver clusters and aromatic rings and their application in the preparation of 1D self-assembled hybrid architectures with ditopic peptide nanotubes. Atomic force microscopy, fluorescence experiments, circular dichroism and computational simulations verified the occurrence of these interactions in the clean and mild formation of a novel peptide nanotube and metal cluster hybrid material. The findings reported here confirmed the sensitivity of silver metal clusters of small atomicity toward noncovalent interactions, a concept that could find multiple applications in nanotechnology. We conclude that induced supramolecular forces are optimal candidates for the precise spatial positioning and properties modulation of molecular metal clusters. The reported results herein outline and generalize the possibilities that noncovalent interactions will have in this emerging field.

Characterization and cytotoxicity studies on liposome–hydrophobic magnetite hybrid colloids

The aim of this study was to highlight the main features of magnetoliposomes prepared by TLE, using hydrophobic magnetite, and stabilized with oleic acid, instead of using the usual hydrophilic magnetite surrounded by sodium citrate. These biocompatible magnetoliposomes (MLs) were prepared with the purpose of producing a magnetic carrier capable of loading either hydrophilic or lipophilic drugs. The effect of different liposome/magnetite weight ratios on the stability of magnetoliposomes was evaluated by monitoring the mean diameter of the particles, their polydispersity index, and zeta potential over time. The prepared magnetoliposomes showed a high liposome-magnetite association, with magnetoliposomes containing PEG (polyethylene glycol) showing the best magnetite loading values. To verify the position of magnetite nanoparticles in the vesicular structures, the morphological characteristics of the structures were studied using transmission electron microscopy (TEM). TEM studies showed a strong affinity between hydrophobic magnetite nanoparticles, the surrounding oleic acid molecules, and phospholipids. Furthermore, the concentration above which one would expect to find a cytotoxic effect on cells as well as morphological cell-nanoparticle interactions was studied in situ by using the trypan blue dye exclusion assay, and the Prussian Blue modified staining method.

Effect of porosity on FMR linewidth of Ln0.67A0.33MnO3 (Ln La, Pr; A Ca, Sr)

Abstract Magnetization and ferromagnetic resonance (FMR) measurements near the Curie temperature have been taken from sol—gel and ceramic samples of Ln 0.67 A 0.33 MnO 3 (Ln  Pr, La; A  Ca, Sr). We demonstrate that the demagnetizing fields arising from the pores in polycrystalline samples cause the FMR line broadening observed below 1.1 T C , with respect to the values of the equivalent single crystals.

Magnetic susceptibility studies in Gd2CuO4 below 300 K

Measurements of the real (χ’) part of the ac magnetic susceptibility have been performed on two polycrystalline Gd2CuO4 samples sintered at different temperatures, 850 °C and 1080 °C in the temperature range 10<T(K)<300. Two well‐defined maxima have been observed in the temperature dependence of χ’. One, located at about 20 K, that is frequency independent. The second is at about 280 K for the sample annealed at 1080 °C, and it is frequency independent. Instead, for the sample annealed at 850 °C the maximum appears at about 210 K, and it is frequency dependent. These results suggest the existence of magnetic domains, with their coherence lengths being a function of the thermal treatment.